The operation of a nuclear reactor results in the creation of various radioactive elements, particularly radioactive isotopes that are present in the cooling air or effluent gases surrounding the reactor. Therefor, the Nuclear Regulatory Commission provides stringent controls for the amount of radioactive components that may be discharged into the atmosphere, or into the water circulated through the cooling towers. The control is normally accomplished by filtering the effluent cooling air to remove any particulate materials. The filtered effluent is then monitored routinely to provide an alert if the permissive amount of radioactive substances present in the effluent gas is exceeded.
In particular, it is desired to closely monitor the amount of radioactive iodine or iodine particulates inasmuch as they pose a substantial health hazard. Molecular levels of radioactive noble gases are discharged into the atmosphere and dissipated at levels that do not pose a significant health hazard inasmuch as the noble gases do not react or combine with organic materials. Radioactive iodine, however, is taken up by organic materials and may be concentrated in the food chain, eventually being concentrated in the thyroid gland. Consequently, radioactive iodine and particulates are much more of a health hazard than radioactive noble gases.
The prior art discloses a number of devices intended to separately monitor the level of radioactive iodine present in a stream of gas containing radioactive noble gases. These detectors do provide an accurate indication of the relative proportion of radioactive iodine present in the gas stream containing radioactive noble gases when operating under normal operating conditions. Examples of these types of monitors are as follows.
U.S. Pat. No. 3,982,129 to Lattin et al. discloses a method and means of monitoring the effluent from nuclear facilities wherein radioactive iodine is detected in the effluent cooling gas from a nuclear reactor by passing the effluent gas through a continuously moving adsorpent filter material which is then purged of noble gases and conveyed to a detector for measuring the retained radioactive iodine adsorped by the filter material.
U.S. Pat. No. 3,731,100 discloses a monitor of the concentration of radioactive iodine in a stream of gas wherein the concentration is measured by counting the total radiation in the effluent, filtering out the particulate radioactive substances in the filter, and then measuring the radiation in the effluent after filteration. The second level of radiation is subtracted from the first level of radiation to obtain an indication of the amount of radioactive iodine retained by the filter.
U.S. Pat. No. 3,953,737 discloses a method and apparatus for monitoring radioactive particles carried by a floating fluid. In this device, the radioactivity of particles suspended in a flowing water stream is continuously monitored by passing the flowing water through a filter to form an increasing collection of the radioactive particulate materials. The collection is continuously monitored by a radioactivity detector which is upwardly adjustable as a function of time. In the event the amount of radioactivity exceeds the predetermined upward adjustment of the counter, an alarm is sounded.
U.S. Pat. No. 4,030,887 discloses a carbon monoxide detection apparatus and method, having a rotatable valve means for alternately directing a sampled stream of carbon monoxide to a first fluid path while simultaneously allowing a second fluid path to discharge to the atmosphere. When the valve means is rotated, the second fluid path receives a fluid flow, and the first fluid path is discharged into the atmosphere.
U.S. Pat. No. 3,997,297 discloses a method and apparatus for detecting a constituent in an atmosphere wherein the incoming air stream is split into two identical fluid paths having plural detectors. A purging gas, preferably argon, is used to remove the non-selected constituents from the region of the detectors. Detection is accomplished by selectively destroying the selected material by heat in one of the fluid paths.
The foregoing detectors, such as that disclosed in U.S. Pat. No. 3,731,100 are sufficiently capable of distinguishing between radioactive iodine and radioactive noble gases under normal design operating conditions. However, the accident at the Three Mile Island Plant in Pennsylvania identified a problem which exists with current on-line radioactive iodine monitors. The problem appears under abnormal or emergency conditions when there are substantially larger amounts of radioactive noble gases passing through the monitor. This problem is particularly accute when the level of radioactive noble gases is on the order of 2-10 orders of magnitude higher than the normal design operating parameters. The monitor may be faced with discriminating at levels one thousand to one million times its intended design parameters.
The additional problem, caused by abnormal conditions, is caused by noble gas atoms which decay as they pass through the detector chamber, emitting gamma radiation. The "Compton Scattering" effect creates an angle of scattered radiation that will impinge upon the window range of the detector. While at normal design parameters, the level is not sufficiently high to be troublesome, when the level of noble gases is increased by one thousand to one million times, the amount of scattering will appear to the monitor, to be radiation emitted within the window range of the detector, and will be detected as radioactive iodine. Therefore, the monitor indicates a much greater amount of radio-iodine in the gas stream than actually exists.